In a train in which two compositions having different composition directions are coupled, a problem that doors indicating the right side in one composition and doors indicating the right side in the other composition are doors on the opposite sides to each other arises. In order to solve the problem, a method of resolving the disagreement in recognition of upstream and downstream directions between communication relay units on a transmission line is disclosed (e.g., see Patent Document 1). Specifically, when compositions having different composition directions are coupled, the recognition of upstream and downstream directions is inversed in all communication relay units within one composition, and thereby, agreement is obtained in recognition of upstream and downstream directions of communication relay units between compositions.
Patent Document 1: JP-A-2005-168126